Cylindrical grating rotation sensor

1) Sensor comprising

• a light source,
• a light detector,
• an internal part,
• an external part,
• at least a first grating arranged at the internal part, and
• at least a second grating arranged at the external part;
• the internal and external parts being arranged so that the first grating and the second grating are facing each other, one of the first and second gratings diffracting light in transmission and the other one diffracting light in reflexion, this sensor being arranged so that the light provided by the light source is first incident on the grating diffracting it in transmission where this light is diffracted into first and second diffraction orders (first diffraction event), then this light diffracted into the first and second diffraction orders is incident on the other grating diffracting the light in reflexion respectively into third and fourth diffraction orders (second diffraction event) and finally the light diffracted back into the third and fourth diffraction orders is incident on the grating diffracting it in transmission where these third and fourth diffraction orders are respectively diffracted again and recombined so as to form a recombined light (third diffraction event), the periods of the first and second gratings as well as the diffraction orders in the first, second and third diffraction events being selected so that two light rays originating from a same first points in the first diffraction event and respectively propagating along first and second directions and then, after the second diffraction event, respectively along third and fourth directions are substantially incident on a same second points of the grating intervening in the third diffraction event where they recombine together in a common fifth direction and interfere, the sensor being further arranged so that the recombined light propagating along the common fifth direction after the third diffraction event is finally at least partially incident on the light detector, the intensity variation of this recombined light being used to determine a relative movement between the internal part and the external part.
• characterized in that,
• the internal part having a first lateral surface with a first grating which surface is globally cylindrical and convex, and the external part having a second lateral surface with a second grating which surface is globally cylindrical and concave, the first and second lateral surfaces both having a same central axis defining a rotation axis for a rotation between the internal part and the external part the angle of which this rotation sensor can measure by means of the intensity variation of the recombined light,
• and the first grating is arranged with its grating lines parallel to the rotation axis, and the second grating is arranged with its grating lines parallel to the rotation axis;
• further characterized in that the Sensor comprises coupling means deviating light coming from the light source in a plane substantially normal to the rotation axis before the first diffraction event and the coupling means are arranged for reflecting the light radially in the plane so that cylindrical waves impinge on the first grating in the first diffraction event, the central axis of these cylindrical waves being aligned on the rotation axis.